The family of Signal Transducers and Activators of Transcription (STATs) plays a central role in signaling by numerous cytokines, polypeptide growth factors, and oncoproteins. STATs were initially described in the context of regulating physiologic cell signaling contributing to such diverse processes as differentiation, proliferation, and apoptosis. A number of studies have implicated STAT activation, particularly STAT3, in transformation and tumor progression. Constitutive activation of STAT3 has been detected in many hematopoietic and solid malignancies, including multiple myeloma, leukemias, lymphomas, mycosis fungoides, as well as carcinomas of the prostate, breast, lung, pancreas, ovary and head and neck (Garcia, R., et al., Oncogene, 20: 2499-2513, 2001; Gouilleux-Gruart, V., et al. Blood, 87. 1692-1697, 1996; Grandis, J. R., et al. Proc Natl Acad Sci USA, 97: 4227-4232, 2000; Huang, M., et al. Gynecol Oncol, 79: 67-73, 2000; and Bowman, T., et al. Oncogene, 19(21): 2474-2488, 2000). Upon activation, STAT proteins dimerize and translocate to the nucleus where they regulate gene expression by binding to specific DNA-response elements (Darnell, J. E., Jr., Science, 277: 1630-1635, 1997).
To directly address the role of STAT3 as an oncogene, a constitutively active mutant of STAT3 was generated (STAT3C) and shown to induce transformation of fibroblasts and tumor formation in nude mice (Yu, C. L., et al., Science, 269: 81-83, 1995 and Bromberg, J. F., et al., Cell, 98: 295-303, 1999). In addition to being a point of convergence for numerous oncogenic signaling pathways, STAT3 also participates in cell growth and survival. One of the first indications that STAT3 signaling contributes to malignancy, at least in part by preventing apoptosis, came from studies showing that increased expression of the anti-apoptotic Bcl-2-family gene bcl-xL is dependent on constitutively activated STAT3 in multiple-myeloma cells (Catlett-Falcone, R, et al., Curr. Opin. Oncol. (1999) 11:490-496). Inhibition of STAT3 signaling blocked the expression of Bcl-xL in these tumor cells and sensitized them to FAS-mediated apoptosis (Catlett-Falcone, R., Curr. Opin. Oncol. (1999) 11:490-496). Consistent with these findings, STAT3 activation has been shown to regulate Bcl-xL expression and apoptosis in a wide range of tumor cells (Grandis, J. et al., Proc Nati Acad Sci USA, 97: 4227-4232, 2000; Bromberg, J. et al., Cell, 98: 295-303, 1999; and Niu, G., et al., Oncogene (2002) 21:2000-2008).
The association of STAT3 activation with transformation and tumor progression suggests that STAT3 is an attractive molecular target for cancer therapy. Several strategies have been used to block the action of STAT proteins, including antisense methods, ectopic expression of dominant-negative mutants (Grandis, J. R., et al., Embo J, 15. 3651-3658, 1996; and Li, L. et al., J. Biol Chem, 277: 17397-17405, 2002), inhibition of upstream kinases (Fry, D. et al., Science, 265: 1093-1095, 1994; Kraker, A. J., et al., Biochem Pharmacol, 60: 885-898, 2000; and Turkson, J., et al., Mol Cell Biol, 19: 7519-7528, 1999), and phosphotyrosyl peptides (Turkson, J., et al., J Biol Chem, 276: 45443-45455, 2001).
An alternative approach to target the action of transcription factors, including STAT proteins, involves the use of double-stranded “decoy” oligonucleotides. The double-stranded DNA decoy closely corresponds to the response element within the promoter region of a responsive gene. By achieving a sufficient concentration of decoy in the target cells, the authentic interaction between a transcription factor and its endogenous response element in genomic DNA is impaired, with subsequent modulation of gene expression (U.S. Patent Publication Nos. 2002/0052333, 2002/0128217 and 2003/0186922 and Nabel, E. G., et al., Science, 249: 1285-1288, 1990).
STAT3 decoys that decrease STAT3 activation and inhibit growth of squamous cell carcinoma of the head and neck (SCCHN), but not normal cells, have been previously described, including, for example, in U.S. Patent Publication No. 2006/0293264, and Leong, P. L., et al., Proc Natl Acad Sci USA, 100: 4138-4143, 2003. These decoys, when injected directly into tumors, have been shown to be effective in preclinical cancer models of the skin, breast, and lung and demonstrated cancer-specific growth inhibition (Romano M F, et al. Leuk Lymphoma 2000; 36:255-62; Chan K S, et al. J Clin Invest 2004; 114:720-8; Xi S, et al. Oncogene 2005; 24:970-9; Zhang X, et al. BMC Cancer 2007; 7:149; and Sun Z, et al. Immunobiology 2006; 211:199-209). Systemic administration of decoy would be preferred over local injection, at least in part because some tumors are not sufficiently accessible and some (such as small metastases) may be clinically undetectable. However, previously known STAT3 decoys are rapidly degraded in human serum, rendering their systemic administration inefficient or ineffective. It is therefore desirable, and an object of the present invention, to provide novel STAT3 decoys with enhanced stability that can feasibly be administered systemically.